题目链接：

Description

Your task is to write a program, which, given two circles, calculates the area of their intersection with the accuracy of three digits after decimal point.

Input

In the single line of input file there are space-separated real numbers x1 y1 r1 x2 y2 r2. They represent center coordinates and radii of two circles.

Output

The output file must contain single real number - the area.

Sample Input

20.0 30.0 15.0 40.0 30.0 30.0

Sample Output

608.366

　　大意是给出两圆心，及圆的半径，求两个圆相交部分的面积，求解关键是对分好case进行处理

Accepted：

1 #include 
     
       2 #include 
      
        3 #include 
       
         4 #define Pi 3.1415926 5  6 using namespace std; 7 typedef struct Point 8 { 9     float x, y;10 }Point;11 typedef struct Circle12 {13     Point p;14     float r;15 }Circle;16 double Dis(Point p1, Point p2);17 double CArea( Circle c1, Circle c2);18 int main()19 {20     float x1, y1, r1, x2, y2, r2;21     cin >> x1 >> y1 >> r1 >> x2 >> y2 >> r2;22     double s = 0;23     Circle c1, c2;24     Point p1, p2;25     if(r1 <= r2)26     {27         p1.x = x1, p1.y = y1, c1.p = p1, c1.r = r1;28         p2.x = x2, p2.y = y2, c2.p = p2, c2.r = r2;29     }30     else31     {32         p1.x = x2, p1.y = y2, c1.p = p1, c1.r = r2;33         p2.x = x1, p2.y = y1, c2.p = p2, c2.r = r1;34     }35 36     s = CArea(c1, c2);37     cout << setiosflags(ios::fixed)<
        
         << s << endl;38     return 0;39 }40 double CArea( Circle c1, Circle c2)41 {42     double s = 0;43     double x = 0, y = 0, h = 0;44     double alpha = 0, beta = 0;45     double d = Dis(c1.p, c2.p);46     if(d >= (c1.r + c2.r))47     {48         s = 0;49         //cout << "case 1" << endl;50     }51     else if(c2.r >= (c1.r + d))52     {53         s = Pi*c1.r*c1.r;54         //cout << "case 2" << endl;55     }56     else if(d >= sqrt(pow(c2.r, 2) - pow(c1.r, 2)) && d < c2.r + c1.r)57     {58         x = (pow(c2.r, 2) - pow(c1.r, 2) + pow(d, 2)) / (2*d);59         h = sqrt(pow(c2.r, 2) - pow(x, 2));60         alpha = asin(h/c1.r);61         beta = asin(h/c2.r);62         s = alpha*pow(c1.r, 2) + beta*pow(c2.r, 2) - d*h;63         //cout << "case 3" << endl;64     }65     else if(d <= sqrt(pow(c2.r, 2) - pow(c1.r, 2)) && d >= c2.r - c1.r)66     {67 68         y = (pow(c2.r, 2) - pow(c1.r, 2) - pow(d, 2)) /(2*d);69         h = sqrt(pow(c1.r, 2) - pow(y, 2));70         alpha = asin(h/c1.r);71         beta = asin(h/c2.r);72         s = (Pi - alpha)*pow(c1.r, 2) + h*y + beta*pow(c2.r, 2) - h*(d+y);73         //cout << "case 4" << endl;74     }75     return s;76 }77 double Dis(Point p1, Point p2)78 {79     return sqrt((p1.x - p2.x)*(p1.x - p2.x) + (p1.y - p2.y)*(p1.y - p2.y));80 }